1. Field of the Invention
The present invention relates to systems and methods for transmitting digital information to a plurality of users, and in particular to a system and method for minimizing interference between spot beam transmissions.
2. Description of the Related Art
The methods by which movies and other media programs that are distributed to theaters for display to audiences have not substantially changed in over 75 years. Cellophane copies of such movies are distributed to each theater in advance of the first showing, and the same copy is repeatedly displayed for audiences until the movie is excessively worn, the license expires, or the theater stops showing the movie in favor of a more popular movie.
There are several problems with this process. Fist, the process of manually and securely distributing physical cellophane copies of each movie is costly. Second, it is time consuming. This is especially important in circumstances where last minute changes must be made to the film before release. Such time concerns often prohibit that any such changes be made.
There is therefore a need for a method and system for distributing media programs in a rapid and inexpensive manner. One possible solution is to transmit digital copies of the movies via a satellite or other high bandwidth medium. However, this solution has its challenges in that the transmission of highly detailed media programs requires a high-bandwidth link between the content provider and the entity displaying the media program.
One technique of increasing the bandwidth is to transmit the data via spot beams. Modern satellites often employ a beam laydown that forms a cellular coverage of a wide geographic area to improve performance. In addition to providing better performance, the narrow beams also allow spatial re-use of the same frequency or time slot, so that the total throughput bandwidth of the satellite can be several times the allocated frequency band. The degree of spatial frequency re-use depends on the carrier-to-interference (C/I) ratio, which is the ratio of the magnitude of the carrier of the desired channel to the magnitude of the interference. In a spot-beam system, the C/I is typically dominated by co-channel interference caused by re-uses of the same frequency and time slot in nearby cells.
The satellite antenna is responsible for ensuring that its transmissions to a given cell do not leak into other cells that re-use the same frequency. The isolation ratio between the weakest desired signal and the strongest interfering signal is typically in the range of 14 to 20 dB when the re-use is as close to the desired cell as it is permitted to be. For example, if the system has a 4 “color” re-use, the nearest re-use of the same frequency could be at the nearest edge of a cell that is centered 2 radii from the center of the desired cell. In this example, a given cell can have 4 nearby re-uses of its frequency, meaning that the aggregate C/I ratio is lower by a factor of 4, or equivalently 6 dB.
Also, in some circumstances a particular user needs more transmitted power from the satellite than another. This differential could be due to weather, obstructions, or the fact that one user is near the center of the cell while the other is at the edge of his cell where the gain of the transmitting antenna is lower. The system accommodates these differences by transmitting the downlink signal of a particular spot beam at higher power than other spot beams, and hence interfering transmission may be at a higher original level than the desired transmission. In spot-beam systems, performance is often limited more by C/I than by C/N (carrier to noise ratio). The fact that the transmitted power differential reduces the worst-case C/I dB for dB is thus a major detriment to system performance.
What is needed is a system and method for minimizing or eliminating such higher-power interfering transmissions. The present invention satisfies this need.